A CR nucleus of energy \(E\) , mass \(A\) , and charge \(Z\)  has interactions with photons and gas at a rate \(\tau ^{-1}_\gamma (E,A)\)  and \(\tau ^{-1}_g(E,A)\)  when propagating in the source environment. These rates are specified by their cross sections, the photon spectral density distribution, and the gas density. Thus we can fully characterize UHECR interactions with photons and gas in a given source environment by knowing the parameters specifying the photon spectrum, and \(\tau _\gamma \)  and \(\tau _g\)  for a reference nucleus and energy. Following [1]}, we adopt 10 EeV \(^{56}\) Fe as this reference.
